Brachyspira pilosicoli (porcine intestinal spirochetosis)
LEVELS: Highly unlikely; Moderate; Routine biosecurity keeps it out; Laboratory-dependent; Minor; Negligible; Some; Needed but not available
| Register id | brachyspira_pilosicoli |
| Type | bacteria |
| Scientific name | Brachyspira pilosicoli |
| NCBI taxid | 52584 |
| Evidence | 3 document(s) |
| Assigned | 2026-09-04, against criteria version 093366e352a2 |
Overview
Brachyspira pilosicoli causes porcine intestinal spirochetosis, a milder cousin of swine dysentery. Weaners, growers and recently mixed pigs develop watery to mucoid diarrhoea, lose condition and take longer to reach market weight; deaths are rare, and the diarrhoea usually settles within two weeks. Not every infected pig scours, but even subclinical animals grow more slowly, which is where most of the cost sits. Under the microscope the spirochaetes attach end- on to the lining of the colon in such numbers that they form a distinctive false brush border. Unlike the dysentery organisms it has a broad host range — people, dogs and birds among others — and it is a confirmed human pathogen, mainly of immunocompromised people and communities with poor sanitation. It is hard to exclude because wild birds carry it and it survives a long time outside the pig: 66 days in cold lake water, 210 days in soil containing pig faeces.
C1 Zoonotic potential
Likelihood of transmission from pigs to humans
Highly unlikely: Human infection with the agent has never been reported, or has been reported but is not attributed to pig or pork exposure
CONFIRMED by Eric. The assignment reached this level from the evidence, and he has reviewed and ratified it.
Confirmed by Eric. CONFIRMED. REVIEWED ON C1_C2_RECLASSIFICATION.md, 2026-09-04, after the C1/C2 rewording that separated exposure from consequence. Eric marked up every moved cell and reversed his own earlier setting on twenty of the twenty-one: "In almost every case, I agreed with new opinion and reversed my old setting." HIS MARKUP HERE WAS "L1". Reverses his 2026-09-02 level 2. hampson_2017 calls zoonotic transmission "highly likely, although it has not been conclusively demonstrated", and every field investigation that identified a probable source found dogs, waterbirds or contaminated water.
Basis. ch45 45.3.3: "Brachyspira pilosicoli colonizes human beings who are usually either immunocompromised or live in developing communities where hygiene is poor and fecal contamination of water supplies may occur." hampson_2017_pilosicoli_review.md, EPIDEMIOLOGY: "Cross-species transmission undoubtedly occurs, and ZOONOTIC TRANSMISSION IS HIGHLY LIKELY, ALTHOUGH IT HAS NOT BEEN CONCLUSIVELY DEMONSTRATED." The supporting experiment is the reverse direction: strain WesB, isolated from an Aboriginal child with diarrhoea, colonised newly weaned pigs and produced watery mucoid diarrhoea and subacute mucosal colitis indistinguishable from that produced by a pig strain. On occupational risk the review says only that "individuals working with intensively farmed pigs, chickens, or other farmed species MAY BE at increased risk of exposure". Where a source has actually been traced it has not been pigs: in Papua New Guinea, among village animals sampled, B. pilosicoli "was detected only in four dogs and a duck and NOT in free-ranging village pigs", and canine and human isolates from the same communities have matched by restriction analysis and PFGE. Carriage in the general population of developed countries is under 1%: fewer than 0.9% of non-Aboriginal Australians, 1.2% of 1,679 Belgian patients with diarrhoea, and 1 of 586 long-term Perth residents.
How this level was decided
MOVED L2 -> L1, re-read 2026-09-04 AGAINST THE REWORDED LABELS. hampson_2017 after 32 pages: zoonotic transmission "IS HIGHLY LIKELY, ALTHOUGH IT HAS NOT BEEN CONCLUSIVELY DEMONSTRATED", and every field investigation that identified a probable source found dogs, waterbirds or contaminated water rather than pigs.
PREVIOUS ANSWER, kept because the evidence behind it has not changed, only the level boundary: Level 2, and the new review sharpens rather than moves it. The level 2 label is "human infection from pigs is plausible but has never (or rarely) been reported", and Hampson says exactly that in his own words -- highly likely, not conclusively demonstrated -- after 32 pages and 228 references. NOTE THE DIRECTION OF THE NEW EVIDENCE, because it cuts against pigs rather than for them: the human-to-pig experiment proves the strains are interchangeable but says nothing about which way transmission runs in the field, and every field investigation that identified a probable source found dogs, waterbirds or contaminated water. The one survey that looked at pigs and people in the same villages found the organism in dogs and a duck and not in the pigs. Under C1 as rescoped to transmission FROM PIGS, that keeps this at level 2 and would not support level 3.
C2 Zoonotic impact
Severity of human illness caused by the agent
Moderate: Clinically important illness is common and may require medical treatment or hospitalization, but death or permanent disability is uncommon
CONFIRMED by Eric. The assignment reached this level from the evidence, and he has reviewed and ratified it.
Confirmed by Eric. CONFIRMED. "c2l3, agree with assessment" EARLIER RULING, 2026-09-02, kept because a re- confirmation does not erase the reasoning it rests on: CONFIRMED. "c2l3, agree with assessment - i have uploaded two papers in inbox, review all criteria for this organism in light of this new info. Only present gaps if they exist, I don't think the papers will change our view but is good evidence none the less."
Basis. ch45 45.3.3: "Human infections have been associated with chronic diarrhea, failure to thrive in children, and irritable bowel syndrome with diarrhea." 45.3.7: "In humans, B. pilosicoli has been isolated from the bloodstream of individuals with severe clinical disease or impaired immunity." ajcp_2023_human_colonic_spirochetosis.md, a systematic review pooling individual patient data from 21 studies, n=113: patients infected with B. pilosicoli were "almost 3 times more likely to have diarrhea, almost 13 times more likely to have fever, 3 times more likely to be HIV positive, 5 times more likely to be immunocompromised" than those infected with B. aalborgi. "The data suggest that B. pilosicoli is an ACUTE SYMPTOMATIC INFECTION (ie, diarrhea, fever) THAT IS USUALLY SUCCESSFULLY TREATED WITH METRONIDAZOLE", which "appeared to lead to symptom resolution in all of the treated patients". hampson_2017_pilosicoli_review.md on the severe end: "A number of patients with a B. pilosicoli spirochetemia have been described. In all cases these individuals have been chronically ill and/or immunocompromised... some have shown multiorgan failure. Spirochetemia with B. pilosicoli is probably uncommon, since a study failed to identify any cases following examination of 1,063 blood samples from patients considered to be potentially at risk."
How this level was decided
Level 3, and the two new papers settle what was previously flagged as a level 2 or 3 judgment. The C2 level 3 label asks for illness that is clinically important and commonly needs medical treatment, without death or permanent disability being common, and that is what the pooled human data describe: diarrhoea and fever severe enough to reach a gastroenterologist and be biopsied, treated with a 10 to 14 day course of metronidazole, resolving in all treated patients. It is not level 2, because "usually mild and self-limiting" does not fit an infection that is characteristically treated rather than waited out, and because chronic diarrhoea with failure to thrive and underweight in Aboriginal children is a lasting harm. It is not level 4, because the life-threatening presentation -- spirochaetaemia with multiorgan failure -- occurs only in people already chronically ill or immunosuppressed, and 1,063 blood samples from at-risk patients produced no cases at all. Eric confirmed level 3 on 2026-09-02 before these papers were read; they support the level he confirmed.
C3 Herd introduction risk
Effort required to keep the agent out of the herd
Routine biosecurity keeps it out: Quarantine of incoming stock, transport and fomite control, cleaning and disinfection, and the usual monitoring reliably exclude it
CONFIRMED by Eric. The assignment reached this level from the evidence, and he has reviewed and ratified it.
Confirmed by Eric. CONFIRMED at level 3 on the C3 review sheet. "l3, as above for B.hyo - I do wonder about moving it to L1 but reality is we have not done systematic prevalence study in US to know if it is essentially in all populations already. We will keep at l3 to be consistent with other pathogenic Brachy's" He raised level 1 himself and declined it for a reason worth keeping: "we have not done systematic prevalence study in US to know if it is essentially in all populations already." Level 1 is a claim about prevalence and there is no US prevalence study to support it. EARLIER RULING, 2026-09-02, kept because a later decision does not erase the reasoning it replaced: CONFIRMED. "c3l5, agree with assessment" RESCALED 2026-09-03, when C3 went from five levels to four and level 4 absorbed level 5. A ruling of his is never overturned by a scale change, so what is recorded here is the same judgment expressed on the new scale. AND THE ENTRY ALSO MOVED, on the pass Eric approved the same day: the whole old level 5 block was re-read against the exclusion test rather than the reservoir test, because twenty-one of those entries had been renumbered past the housing level on 2026-09-01 without being judged against it. See the entry justification for the reason this one moved. Recorded on the old scale as level 5, from_level 5.
Basis. ch45 45.3.10.2: "It can be difficult to avoid introducing B. pilosicoli into herds because of the presence of reservoir hosts such as wild birds." hampson_2017_pilosicoli_review.md, EPIDEMIOLOGY: the spirochete "may survive for prolonged periods in natural water systems such as lakes and ponds", remaining viable in lake water for 66 days at 4C, "for 119 days in soil and for 210 days both in soil containing 10% pig feces and in pig feces alone". Infection "may be introduced into naive herds by carrier pigs or by feral birds or animals that access the farm"; on one farm the organism was recovered from chickens, effluent pond water and the wild ducks visiting that pond, with a pond isolate genetically identical to a pig isolate. The eradication precedent and its limit: "B. pilosicoli has been eradicated from a 60-sow pig herd by treatment of all animals with tiamulin followed by relocation of the breeding herd, thorough cleaning and disinfection of the original premises, and then returning the adult animals to the farm. THIS PROTOCOL WOULD BE MORE DIFFICULT TO FOLLOW IN LARGER HERDS, AND THE EXISTENCE OF RESERVOIR HOSTS SUCH AS WILD BIRDS AND RODENTS PRESENTS AN ONGOING THREAT OF REINTRODUCTION."
How this level was decided
MOVED L5 -> L3 on 2026-09-03, when C3 was reduced to four levels and the whole level 5 block was re-read against the exclusion test rather than the reservoir test. Eric: "C3 is about exclusion, not reservoirs per se, though the two are related", and "who cares about reservoirs? Reservoirs really don't matter as long as you keep domestic pigs from becoming infected through biosecurity implementation." THE PREVIOUS LEVEL WAS NEVER JUDGED: this entry carried a RENUMBERED L4 -> L5 stamp from 2026-09-01, meaning it was moved past the newly created housing level rather than assessed against it. with hampsonii and suanatina, and for the same reason: the waterfowl route is closed by housing, the pig-to-pig route is not, and level 3 is where the routine programme lives.
EARLIER REASONING, kept because it is the evidence read: Level 5, and the review states the level 5 test directly. The axis is the effort required to exclude, and level 5 is where a reservoir sits outside the farm and the industry and no action removes it. Wild birds and rodents that access farms are precisely that: the organism has been eradicated from a herd, which shows the barrier is not impossible, but the review says in the same paragraph that reinfection remains an ongoing threat because the reservoir is still there. NOTE THAT PERMANENCE IS THE TEST AND NOT PENETRATION, per the standing note: waterbirds do not walk into a barn, they contaminate the water, which is why the review says water should not be recycled on piggeries. The survival figures are what make the reservoir persistent rather than transient -- 210 days in pig faeces and 119 days in soil.
C4 Detection difficulty
Difficulty of recognizing and confirming infection
Laboratory-dependent: The presentation does not point to this disease specifically, but local or regional laboratories can confirm it with a validated assay once it is suspected
CONFIRMED by Eric. The assignment reached this level from the evidence, and he has reviewed and ratified it.
Confirmed by Eric. CONFIRMED. "c4l2, agree with assessment"
Basis. ch45 45.3.8: "PIS/PCS represents the most difficult differential diagnosis, as it can closely resemble mild cases of SD." hampson_2017_pilosicoli_review.md, DIAGNOSIS: primary isolation needs selective solid medium -- "Trypticase soy agar supplemented with 5 to 10% defibrinated ovine or bovine blood, together with 1 to 5 antibiotics (including spectinomycin, rifampin, spiramycin, vancomycin, polymyxin, and/or colistin)" -- incubated anaerobically, and "plates should be examined after 3 days and reexamined every 2 to 3 days for UP TO 2 WEEKS". "Colonies generally are not formed", so identification rests on the sheen of confluent growth plus dark-field or phase-contrast morphology, and "it is preferable to undertake repeated rounds of subculturing to obtain a pure culture before proceeding with other tests". Species confirmation is by PCR.
How this level was decided
Level 2 stands, and the new detail is about effort rather than about validity. The C4 axis is whether a validated assay exists and where it can be run: species-specific PCR exists and is used routinely, and the selective culture is standard veterinary diagnostic laboratory work rather than reference or research work, so this is not level 3. What keeps it off level 1 is unchanged -- loose faeces resembling wet cement in a recently weaned or recently mixed pig has one of the longest differentials in the chapter, and the disease it most resembles is mild swine dysentery. WORTH KNOWING FOR TURNAROUND rather than for the level: a negative culture is not final for up to two weeks, and the organism forms no colonies, so a laboratory that does not routinely look for Brachyspira will not find it incidentally.
C5 Production cost
Financial impact on the infected farm's cost of production
Minor: Small and generally short-lived losses with little effect on overall cost of production
CONFIRMED by Eric. The assignment reached this level from the evidence, and he has reviewed and ratified it.
Confirmed by Eric. CONFIRMED at level 2 on the 2026-09-02 re-read. He wrote "c1l2" against this cell, which names C1; the cell is C5, the level he wrote is the level the cell is on, and the prose is about the cost of the disease, so it is read as c5l2. His words: "agree with assessment - to my knowledge there are not good estimates of the cost of this disease so we will leave at l2 as best guess." EARLIER RULING, 2026-09-02, kept because a re-confirmation does not erase the reasoning it rests on: CONFIRMED. "c5l2, agree with assessment"
Basis. ch45 45.3.1: "Pigs with PIS/PCS exhibit variable loss of condition, which leads to increased time to reach market weight and disrupts efficient production flow." 45.3.6: "Not all infected animals develop diarrhea, but subclinical infections may still depress growth rates." "Diarrhea is usually self-limiting and lasts 2-14 days." "Between 17 and 100% of pigs may become infected, with 17-67% developing diarrhea and 8-100% having colitis. Mortality is rare in the field." 45.3.10.1: treatment and control are "modeled on procedures developed for SD, although modifications can be made because of the milder economic impact of PIS/PCS", and 45.3.10.3: "the economic impact generally does not warrant such costly procedures" as eradication.
How this level was decided
Level 2, and the chapter makes the comparison for us twice -- the economic impact is milder than swine dysentery, and it does not warrant the eradication programmes SD justifies. Mortality is rare, the diarrhoea is self-limiting within two weeks, and what remains is lost condition and delayed market weight. Small and generally short-lived losses. FLAGGED as a level 2 or 3 judgment, because the subclinical half is not short-lived: the chapter says infection depresses growth rates even without diarrhoea, and up to 100% of a group may be infected. Delayed days to market across a whole barn is a measurable increase in cost of production.
C6 Market impact
Duration of material disruption to pork and pig markets
Negligible: Little or no material disruption to supply or demand when disease occurs on one or more farms
CONFIRMED by Eric. The assignment reached this level from the evidence, and he has reviewed and ratified it.
Confirmed by Eric. CONFIRMED. "c6l1, agree with assessment" EARLIER RULING, 2026-09-02, kept because a re- confirmation does not erase the reasoning it rests on: CONFIRMED. "c6l1, no evidence to suggest market is or would react to a diagnosis"
Basis. Section 45.3 records no trade measure, movement control or consumer response. "PIS/PCS has been reported in most pig-producing countries" and recognition is increasing as antimicrobial growth promoters are withdrawn.
How this level was decided
INFERRED. Under the standing note SCORING/market_impact this is an agent reported from most pig-producing countries, endemic and routinely diagnosed in the US, with no market move on record. Level 1.
C7 Treatment potential
Potential for treatment to improve outcomes
Some: Treatment exists but is inconsistent, requires extralabel use, or works only in some circumstances
CONFIRMED by Eric. The assignment reached this level from the evidence, and he has reviewed and ratified it.
Confirmed by Eric. CONFIRMED. "c7l2, agree with assessment"
Basis. ch45 45.3.10.1: "tiamulin, valnemulin, carbadox, dimetridazole, and, to a lesser extent, lincomycin have had low MIC values when tested against porcine B. pilosicoli isolates." hampson_2017_pilosicoli_review.md, TREATMENT: "The main antimicrobials used to treat PIS in different countries are tiamulin, valnemulin, carbadox, dimetridazole, and lincomycin, although isolates that are resistant to one or more of these drugs occur." "The susceptibility patterns of different isolates vary, and hence IT IS RECOMMENDED THAT ISOLATES BE TESTED FOR SUSCEPTIBILITY PRIOR TO SELECTION OF THE DRUGS TO BE USED FOR TREATMENT." Resistances to macrolides, lincosamides and pleuromutilins are attributed to mutations at the 50S ribosomal binding site, and penicillin resistance to a wide array of novel group D beta-lactamases. Some previously effective drugs, "such as dimetridazole and carbadox, have been prohibited from use in production animal species in many countries". The US tiamulin label names "swine dysentery associated with Brachyspira (formerly Serpulina or Treponema) hyodysenteriae susceptible to Tiamulin" and names no other Brachyspira species.
How this level was decided
Level 2, AND THE REASON IS CORRECTED HERE. The previous justification said effective drugs "exist and are labelled", which is wrong on the test Eric set on 2026-09-02: the US tiamulin label names B. hyodysenteriae and no other Brachyspira, so treating B. pilosicoli with it is extralabel, exactly as for hampsonii and suanatina. That alone puts the cell at level 2 under the standing note on treatment_potential, independent of how well the drug works. The pharmacology points the same way rather than against it -- susceptibility varies enough between isolates that the review recommends testing before choosing a drug, named resistance mechanisms exist for all three drug classes in use, and two of the five drugs it lists have been prohibited in food animals in many countries. Treatment exists and is used; it is neither labelled for this species nor dependable without testing.
C8 Vaccine availability
Availability of effective vaccines or bacterins
Needed but not available: No effective vaccine is available in the US or has been developed, and the disease would justify vaccinating if one existed
CONFIRMED by Eric. The assignment reached this level from the evidence, and he has reviewed and ratified it.
Confirmed by Eric. CONFIRMED. "c8l3, agree with assessment and my reasons above" EARLIER RULING, 2026-09-02, kept because a re-confirmation does not erase the reasoning it rests on: CONFIRMED. "c8l3, agree with assessment and another good example of the powerpoint note we made earlier. sometimes returns on investment for vaccines or treatment comes when you don't expect it. Disease is at a low, chronic level in most US herds i suspect and because pigs aren't dying, tend to just ignore or do prophylactic treatment. probably don't know true cost of disease until you better control it and understand living without it"
Basis. ch45 45.3.10.2: "Unfortunately, no effective vaccines are available for B. pilosicoli. An autogenous bacterin induced systemic antibody titers, but the pigs still became colonized and developed diarrhea after experimental challenge." hampson_2017_pilosicoli_review.md, PREVENTION: "NO COMMERCIAL VACCINES ARE AVAILABLE for prevention of B. pilosicoli infections. In pigs an autogenous bacterin induced systemic antibody titers, but the vaccinated animals still became colonized and developed diarrhea after experimental challenge. In mice, experimental vaccines based on two recombinant oligopeptide-binding proteins... Oligopeptide-binding proteins may have potential for use as B. pilosicoli vaccine components; however, they require further testing in mice and other animal species." Prevention otherwise rests on management: limiting mixing of ages and sources, all-in/all-out in place of continuous flow, avoiding abrupt diet changes, and cleaning and disinfection.
How this level was decided
Level 3, and the review is a second independent statement of the same finding from the author of the chapter. This is a genuine gap rather than a product nobody built: an autogenous bacterin WAS built, it raised systemic antibody, and the vaccinated pigs were still colonised and still scoured on challenge. The only candidates beyond it are two recombinant oligopeptide-binding proteins tested in mice and needing further work in other species. NOTE THE CONTRAST WITH E. COLI NON-ENTERIC INFECTION, where Eric moved C8 to level 1 on 2026-09-02 because the autogenous route addresses the wrong organism and the disease does not warrant a vaccine: here the autogenous route was aimed at the right organism, failed on challenge, and the disease is common enough in US herds that he explicitly wanted the gap counted -- "sometimes returns on investment for vaccines or treatment comes when you don't expect it... probably don't know true cost of disease until you better control it".
Levels and evidence are generated from data/assignments/brachyspira_pilosicoli.yml; the overview is authored in data/overviews/brachyspira_pilosicoli.md. Do not hand-edit this page — corrections go in data/assignments/CORRECTIONS.yml.
Quoted material marked Basis is from Zimmerman JJ, Karriker LA, Ramirez A, Schwartz KJ, Stevenson GW, Zhang J, eds. Diseases of Swine, 12th edition. Hoboken: Wiley Blackwell, 2025 — except where another source is named in the quotation itself.